It is very important to point out that while the Start Button has returned to the place where it has live on the desktop since 1995 it does not function the same way in terms of opening a Start Menu. The Start Menu essentially is still dead.
The “new” Start Button allows users to click what they have been used to clicking for decades in order to access their apps but instead of opening the Start Menu it simply takes you back to the Start Screen. The same thing could be accomplished by pressing the Windows button on the keyboard or by tapping or clicking the Start icon on the charms menu. Again, it just allows users to have the familiar icon to click on.
The real value to this new button however is the right click. Right clicking on the Start Button brings up an improved system menu. You could access this same menu in Windows 8 by pressing Windows-X. The new system menu includes the option Shut Down or Sign Out, which expands into the familiar Sign Out, Sleep, Shut Down, and Restart options. This is an improvement because now with the mouse, I can log out or I can reboot like I used to by clicking on the Start Button without having to get the charms menu up or go back to the Start Screen or lock screen. I think this will make it easier for desktop users to make the switch to Windows 8 as a familiar routine has returned.
The real treachery though is that the use of the Start Button is not consistent throughout the Windows 8.1 experience. It only appears when running the desktop or running desktop apps. It does not appear on the Start Screen or when running any of the Start Screen apps. In that experience, you have to use the Windows key or the charms menu. That means the user has to be more aware of what mode they are using the hardware in – for some users that isn’t a big deal but for others it may be very confusing. Think about your grandma.
A unified operating system experience should be unified, not just between devices but also between operating modes for the same device.